FIG. 18 is a block diagram for explaining the arrangement of a general alignment mechanism. FIG. 19 is a view showing a wafer on which an alignment mark is formed. FIG. 20 is a flow chart for explaining general alignment processing procedures. Wafer alignment by a general semiconductor manufacturing apparatus will be described with reference to these drawings.
If a wafer W is supplied to the semiconductor manufacturing apparatus, a mechanical alignment apparatus MA mechanically aligns the wafer W by using the periphery of the wafer W and an orientation flat or notch (notch N is shown in FIG. 19) to determine the rough position of the wafer W (step S02). The mechanical alignment precision is about 20 μm. Then, the wafer is set on a chuck CH by a wafer supply apparatus (not shown) (step S03), and pre-aligned (step S04). In pre-alignment, a mirror MM is inserted into an optical path formed by an alignment light source Li and mirror M1 in a scope SC. The mirror MM guides alignment light to a sensor S1 set to a low magnification. In pre-alignment, left and right pre-alignment marks PAL and PAR shown in FIG. 19 are detected using the low-magnification sensor S1, and their mark positions are obtained to attain the center of the wafer. The alignment precision in this pre-alignment is about 4 μm.
Finally, global alignment is performed to accurately obtain the position of the wafer W and the position of an exposure shot (step S05). In global alignment, the mirror MM is removed from the optical path in the scope SC. A sensor S2 set to a high magnification is used to measure the positions of global alignment marks FX1 to FX4 and FY1 to FY4 on the wafer W shown in FIG. 19. Global alignment provides X- and Y-direction shifts of the wafer W, the rotational component, and the magnification component of the shot array. The global alignment precision must be 50 nm or less in a machine which manufactures current 256-Mbit memories.
The scope SC shown in FIG. 18 will be explained with reference to FIG. 21. FIG. 21 is a view showing the schematic arrangement of the scope SC.
In FIG. 21, light guided from an illumination light source 401 (fiber or the like) passes through a switching ND filter 415 serving as a beam attenuation means. Then, light is guided to a polarization beam splitter 403 (corresponding to the mirror M1 in FIG. 18) via an illumination optical system 402.
The switching ND filter 415 is made up of ND filters (415a to 415f) having a plurality of discrete transmittances. A desired ND filter can be used by driving a rotation driving system 420. A controller 421 controls the rotation driving system 420 to select an ND filter so as to optimize the brightness in accordance with the reflectance of an object to be observed.
The filter is not limited to the switching ND filter. The same structure can also be constituted by rotation of a polarizing plate for a light source such as an He—Ne laser which emits light having a uniform polarization characteristic.
S-polarized light reflected by the polarization beam splitter 403 to a direction perpendicular to the sheet surface of FIG. 21 passes through a relay lens 404 and λ/4 plate (¼-wavelength plate) 409. After that, light is circularly polarized and Kohler-illuminates an alignment mark AM formed on a wafer 6 via an objective lens 405.
Reflected light, diffracted light, and scattered light from the alignment mark AM return through the objective lens 405 and λ/4 plate 409, and are converted into P-polarized light parallel to the sheet surface of FIG. 21. P-polarized light passes through the polarization beam splitter 403, and forms the image of the alignment mark AM on a photoelectric conversion element 411 (413) (e.g., CCD camera) via an imaging optical system 410 (412). The position of the wafer 6 is detected based on the position of the photoelectrically converted alignment mark image.
The imaging optical systems 410 and 412 will be described. The switching mirror 414 (corresponding to the mirror MM in FIG. 18) which switches an optical path is interposed between the polarization beam splitter 403 and the imaging optical system 410 (412). The switching mirror 414 is inserted into an optical path to guide light to the imaging optical system 412 having a low magnification, which allows observing the alignment mark AM on the wafer at low magnification in a wide region. The switching mirror 414 is removed from the optical path to guide light to the high-magnification detectable imaging optical system 410. The high-magnification imaging optical system 410 makes it possible to detect the alignment mark on the wafer at high precision in a narrow region.
The controller 421 acquires a wafer position on the basis of information about the photoelectrically converted alignment mark image. The controller 421 sets an optimum one of the ND filters 415a to 415f by issuing a command to the rotation driving system 420 so as to optimize the light quantity in accordance with the brightness and contrast of the alignment mark AM.
At this time, to detect the alignment mark AM on the wafer 6 at high precision, the image of the alignment mark AM must be clearly detected. In other words, the SC must be focused on the alignment mark AM. For this purpose, an AF detection system (not shown) is generally constituted. The alignment mark is driven to the best focus plane of the SC on the basis of the detection result of the AF detection system, thus detecting the alignment mark.
As described above, accurately obtaining the wafer position requires at least pre-alignment and global alignment on the chuck. In pre-alignment, the mark must be detected in a wide field of view after rough alignment by mechanical alignment. The mark must be detected by a low-magnification scope and must be as large as about 100 μm. In global alignment, the mark is precisely detected by a high-magnification scope because the mark has already been aligned with a shift of about 4 μm by pre-alignment.
While a plurality of detection systems for low magnification (pre-alignment) and high magnification (global alignment) are required, demands are arising for short-time detection and measurement. Since the number of wafers processed per unit time must be increased, the time of processing called alignment not accompanied by exposure must be shortened as much as possible.
A general scope SC used for alignment must drive the switching ND filter 415 for an optimum light quantity, and adjust the light quantity to an optimum one (light control) every high/low-magnification detection. The frequency at which the switching ND filter 415 is switched increases, further decreasing the throughput of exposure processing.